.gitignore: ignore *.lz4 files

Now that lz4 kernel compression is available, add *.lz4 to .gitignore.

Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Acked-by: Kyungsik Lee <kyungsik.lee@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>